Why is aws lambda invocation client incorrectly returning ClientExecutionTimeoutException?

Javadocs in aws-sdk-java says:

 For functions with a long timeout, your client might be disconnected during synchronous invocation while it waits for a response. Configure your HTTP client, SDK, firewall, proxy, or operating system to allow for long connections with timeout or keep-alive settings.

On the other hand, previously AWS Lambda was limited up to 5 minutes, later this limit was increased up to 15 minutes.

I would check:

1. The client sdk version is up to date
2. The connection is not closed by your network
3. Move to an async invocation via AWSLambdaAsyncClient.invokeAsync() for long running invocations.

I've accepted Ezequiel's answer since it was technically a networking / OS issue, but here is a more detailed result:

We had to ensure all relevant clients were configured to keep alive tcp connections. We then had to add these properties to the /etc/sysctl.conf file on our EC2 residing in a private subnet because the NAT gateway is set to kill idle connections beyond 350s:

net.ipv4.tcp_keepalive_time = 300
net.ipv4.tcp_keepalive_intvl = 100
net.ipv4.tcp_keepalive_probes = 6